Prosthetic heart valves may be used to replace diseased natural heart valves in human patients. Mechanical heart valves typically have a rigid orifice ring and rigid hinged leaflets coated with a blood compatible substance such as pyrolytic carbon. Other configurations, such as ball-and-cage assemblies, have also been used for such mechanical valves.
In contrast to mechanical heart valves, bioprosthetic heart valves comprise valve leaflets formed of biological material. Many bioprosthetic valves include a support structure, or stent, for supporting the leaflets and maintaining the anatomical structure of the valve. Stented bioprosthetic valves generally are prepared in one of two ways. In a first method of preparation, a complete valve is obtained from either a deceased human or from a slaughtered pig or other mammal. Human valves or valve components implanted into a human patient are referred to herein as a "homografts," while the corresponding animal valves or valve components are termed "xenografts." In the case of homografts, the retrieved valve typically is treated with antibiotics and then cryopreserved in a solution of nutrient medium (e.g., RPMI), fetal calf serum and 10% DMSO. In the case of xenografts, the retrieved valve is trimmed to remove the aortic root, and the valve is chemically cross-linked, typically in a glutaraldehyde solution. The cross-linked valve is then attached to a stent. The stent provides structural support to the valve and, with a sewing cuff, facilitates attachment of the valve to the patient by suturing. In a second method of preparation, individual valve leaflets are removed from a donor valve or are fashioned from other sources of biological material, e.g., bovine pericardium. The individual leaflets are then assembled by suturing the valve leaflets both to each other and to the stent. When bovine pericardium is used, the valve (trileaflet or bileaflet) is fashioned from one piece of pericardium. The material is then draped on the stent to form the "cusps."
One of the major functions of stents is to serve as a framework for attachment of the valve and for suturing the valve into place in the human patient. Toward that end, stents are frequently covered with a sewable fabric, and have a cloth sewing or suture cuff, typically an annular sewing ring, attached to them. The annular sewing ring serves as an anchor for the sutures by which the valve is attached to the patient. Various stent designs have been implemented in a continuing effort to render valve implantation simpler and more efficient. Inevitably, however, a stent limits interactions with aortic wall dynamics and tends to inhibit natural valve movement. This results in post-operative transvalvular gradients with resultant additional work burden on the heart. In addition, a stent causes a reduction in size of the bioprosthetic valve that can be placed in a particular location, since the stent and sewing cuff occupy space that otherwise would be available for blood flow.
Stentless valves have demonstrated better hemodynamic function than stented valves. This is because stentless valves are sewn directly into the host tissues, without the need for extraneous structure such as a sewing cuff. Such extraneous structures inevitably compromise hemodynamics. Stentless valves closely resemble native valves in their appearance and function, and rely upon the patient's tissues to supply the structural support normally provided by a stent. The main disadvantage to stentless valves has been in their difficulty of implantation. Stentless valves require both inflow and outflow suturing, and physicians qualified to implant stented valves can lack the surgical training and experience required for implantation of stentless valves.
Some bioprosthetic valve manufacturers have attempted to develop methods and materials to ease the implantation of stentless valves, including holders, different suturing techniques or suturing aids. None of these approaches has significantly shortened implant times without adversely affecting valve performance.
Stents for bioprosthetic heart valves have been formed from a variety of non-resorbable materials including metals and polymers. With non-resorbable materials, the long-term fatigue characteristics of the material are of critical importance. Unusually short or uneven wear of a stent material may necessitate early and undesirable replacement of the valve. The selected material must also be biocompatible and have the desired stress/strain characteristics.
Various biodegradable materials have been suggested or proposed for use with vascular or non-vascular implants. For example, Goldberg et al., U.S. Pat. No. 5,085,629 discloses a biodegradable infusion stent for use in treating ureteral obstructions. Stack et al., U.S. Pat. No. 5,306,286 discloses an absorbable stent for placement within a blood vessel during coronary angioplasty. Duran, U.S. Pat. No. 5,376,112 discloses an annuloplasty ring to be implanted into the heart to function together with the native heart valve. Duran suggests (Col. 6, lines 6-8) without further elaboration that the annuloplasty ring could be fashioned of resorbable materials.
The prior art stents are designed primarily to maintain a fluid flow patency for a selected period of time. These stents are not designed to support a secondarily functional tissue such as a valve apparatus. Thus, the prior art does not teach or suggest that heart valve stents, with their particular configuration and stress/strain requirements, could be fashioned of bioresorbable materials.